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Imagine a gyroplane, if something happen mid-flight (when the plane is basically an autogyro and the engine provides no power to the helicopter rotor) and just the wind generated by forward flight can no longer turn the helicopter rotor enough to provide enough lift, could the engine partially take over? Like using the engine to provide just enough power, with the rest provided by the wind, so the helicopter rotor could spin enough?

I don't want the plane to autorotate downward. I want the plane to keep flying, but using the engine to increase the rpm. If the incoming wind could turn the top rotor in a certain rpm, could we increase the rpm using the engine? In my memory, rotodyne could do this because it uses a tip jet, but what if we use normal piston or turbine engine?

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  • $\begingroup$ Are you talking about in a hover, or very low forward speed such that you can't auto rotate? I suppose you could come up with some kind of clutching mechanism, but I'm interested in why auto rotate isn't an option? $\endgroup$ – Ron Beyer Jan 30 at 2:44
  • $\begingroup$ What would happen other than the engine dying? $\endgroup$ – Eric S Jan 30 at 4:02
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The problem is that as an autogyro, the air is going through the rotor from below, and as a helicopter, the air is coming from above and it has to be one or the other.

In any case, gyroplanes don't have a problem of low rotor rpm as long as you keep the rotor loaded (and you always keep the rotor loaded). If you slow down, the rotor rpm doesn't decay, the machine just descends steeper. You can slow a gyro to zero airspeed, and it will maintain most of the rotor rpm on its own and autorotate straight down, although the vertical speed may get pretty high. If you have a landing gear that can tolerate a 500 fpm descent, and start from only 20-30 feet or so, you can land like that. This is why some small autogyros don't even bother with a rotor rpm gauge.

So to get low rotor rpm, you'd have to unload it to take away the aerodynamic forces driving the blades around by pitching over to go zero G or close to it, and at that point an engine might be useful to keep the rpm from dropping, but you'll have probably already chopped the tail off from the blades flapping down (low G maneuvers in gyros are to be avoided at all costs - as with 2 blade helicopters, for different reasons), and you'll already be on your vertical trip to your final resting place anyway.

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  • $\begingroup$ It's true that the rotor spins at a more or less constant angular velocity at normal flight airspeeds, but if the forward speed falls to zero and the machine enters a vertical autorotation, the rotor revs fall by about 10%... In the classic gyros of the 1920s and 30s, you could autorotate to the ground, vertically, and survive the landing, but modern gyroplanes can reach 1500 fpm in vertical autorotation, and that's too much... $\endgroup$ – xxavier Jan 30 at 10:10
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    $\begingroup$ Why not just talk about autorotation with helicopters. Don't they fully disengage the drive and "glide" down to land? In that case the wind can turn the rotor. $\endgroup$ – Robert DiGiovanni Jan 30 at 12:22
  • $\begingroup$ But the wind has to reverse direction through the rotor when the helicopter turns into a gyro glider. You regulate rpm n a helicopter gliding with collective pitch changes. His question is about auto gyros anyway. $\endgroup$ – John K Jan 30 at 13:50
  • $\begingroup$ @xxavier I don't think you watched the video linked. The vertical sink rate is somewhere in the range of 500 fpm. $\endgroup$ – John K Jan 30 at 13:51
  • $\begingroup$ @John K It's a very light machine, the 'Monarch'; and those vertical drops are from 15-20 meters or so only. The gyro hasn't enough altitude to accelerate, in vertical autorotation, to the very high terminal speed it would reach. At a prudent altitude, I myself have experimented vertical autorotations with the prop at idle in a conventional gyro (ELA-115) and got scared (and recovered) when the sink speed passed the 1000 fpm mark... I've written those 1500 fpm 'off the top'. I'm not sure now, but I think it's the calculated terminal speed, for a gyro of 400 kg and a rotor dia. of 9 m... $\endgroup$ – xxavier Jan 30 at 14:21
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No, the rotor's power is (in flight) never "by engine." Whenever such an aircraft is not touching the ground, its rotor is fully in what helicopterists would call autorotation.

Gyroplane, autogyro, and gyrocopter mean the same thing, depending on what part of the world you're in. In flight the rotor is unpowered. It may be powered before takeoff for convenience, but not in flight. The engine provides only forward thrust, through a conventional propeller, not through the rotor.

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Most present-day gyros have a two-blade teetering rotor that has to be kept always under load, since it is the airflow associated with that load what keeps the rotor spinning and the blades rigid. If –as happens with zero-G maneuvers– the rotor revs fall below a certain value, the blades lose their 'centrifugal rigidity', and they may flap out of control, chopping the tail and the propeller. It has happened more than once.

It could be interesting to have a direct mechanical connection between the engine and the rotor, so that it could be kept revving above the critical level even at zero load. With a degree of forward airspeed, the torque reaction can be contained with the rudder. That 'partial rotor power' has been tried in a few experimental machines gyroplanes in order to attain shorter takeoff runs.

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